Question

What is the action of the Nessler’s reagent on ammonia?
A. brown ppt.
B. blue ppt.
C. black ppt.
D. green ppt.

Answer 1

Hint: Nessler’s reagent is used for the qualitative test of ammonia. The colour of precipitate after the reaction of Nessler’s reagent with ammonia will be the colour of the complex formed.

Complete step by step answer:
Let us check what Nessler’s reagent is. Potassium tetraiodomercurate (II) (${{\text{K}}_{\text{2}}}{\text{[Hg}}{{\text{I}}_{\text{4}}}{\text{]}}$) is used as Nessler’s reagent. It is an inorganic compound consisting of potassium cations and the tetraiodomercurate (II) anion. On being reacted with gaseous ammonia, it produces brown fumes and on being passed through a solution of ammonia, it gives a dirty brown precipitate. The reaction is as follows:
${\text{2}}{{\text{K}}_{\text{2}}}{\text{[Hg}}{{\text{I}}_{\text{4}}}{\text{] + 3KOH + N}}{{\text{H}}_{\text{3}}} \to {\text{[OH}}{{\text{g}}_{\text{2}}}{\text{.N}}{{\text{H}}_{\text{2}}}{\text{]I + 7KI + 2}}{{\text{H}}_{\text{2}}}{\text{O}}$
The product formed after the reaction of ammonia and Nessler’s reagent is brown in colour. Therefore, the correct option is (a), brown precipitate.

Additional Information:
-Nessler’s reagent is named after the German chemist Julius Nessler.
-Nessler’s reagent is a pale yellow solution.
-We already studied that the Nessler’s reagent is used for qualitative analysis of ammonia. So, what is basically qualitative analysis! In qualitative analysis, the chemical properties of an unknown substance are determined by systematically reacting the unknown with a number of different reagents.
-Here, we are determining the chemical properties of ammonia by systematically reacting the ammonia with the Nessler’s reagent.
-You have to know that qualitative analysis is different from quantitative analysis.
-Qualitative analysis can tell you whether an atom, ion, functional group, or compound is present or absent in a sample, but it doesn’t provide information about its quantity. Quantification of a compound is done by quantitative analysis.

Note: You have to remember the chemical formula of Nessler’s reagent and the complex formed after the reaction with ammonia. This complex is responsible for the brown precipitate. Don’t be confused with the colour of Nessler’s reagent which is pale yellow and the colour of complex is brown.